Characterizing Drought for Forested Landscapes and Streams

The changing nature of drought is a growing global concern (Cook and others 2015, Dai 2011, Seneviratne and others 2010, Sheffield and Wood 2008b, Trenberth and others 2014, Wilhite and others 2014). Drought can be a severe natural disaster with substantial social and economic consequences affecting large areas with extended durations (Wilhite and Buchanan-Smith 2005). Although it is clear that shifts in circulation patterns, energy for evapotranspiration, and air temperatures are changing in ways that enhance the consequences of drought, there is only weak consensus about the effects of climate change on drought occurrence (IPCC 2013, Seneviratne and others 2012, Trenberth and others 2014). Some of that uncertainty stems from the complex nature of quantitatively defining drought, but also because some of the changes in drought characteristics are only partially reflected in traditional drought metrics [Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI)]. Furthermore, although these traditional metrics have adequately reflected the consequences of meteorologically derived moisture deficits on agricultural commodities and water supply, there is a poorer (although improving) understanding of how drought interacts with forests and rangelands and their associated aquatic habitats. Understanding the potential impacts of future drought on forests and rangelands requires knowledge of how droughts impact forest, shrub, and rangeland structure (covered in other chapters in this assessment) and how drought projections are characterized in the General Circulation Model (GCM) output. 
The purpose of this chapter is to explore drought as a hydrometeorological phenomenon and reflect broadly on the characteristics of drought that influence forests, rangelands, and streams. It is a synthesis of understanding about drought processes, hydrology, paleoclimatology, and historical climate variability, and how this understanding can help predict potential future droughts and their consequences to forests and rangelands. It describes alternative approaches for characterizing drought and highlights additional work that could inform projection and adaptation for future droughts.